The role of the working medium is not only used in the hydraulic pressure, and ensure the machine working parts work sensitive, reliable, long life and less leakage. Hydraulic press for the basic requirement of working medium is: (1) there are appropriate liquidity and low compressibility, in order to improve the transmission efficiency; (2) can prevent rust; 3. Have good lubrication performance; (4) easy to seal; (5) stable performance and long work without deterioration. Hydraulic press the original water as working medium, after the switch to join a small amount of emulsified oil emulsion in water, in order to increase lubricity and reduce the rate of corrosion. During the late 19th century in the mineral oil as working medium of hydraulic press.

Oil has good lubricity, anticorrosive and moderate viscosity, to improve the performance of the hydraulic press. In the second half of the 20th century appeared a new kind of water-based emulsion, the emulsion form is "invert", instead of the original "oil-in-water". "Water-in-oil emulsion's minister for oil and its lubricity and corrosion is close to the oil, and oil content few, is not easy to burn. But the price of water-based emulsion more expensive, restricts the promotion.